As detailed in another thread, the rollout is really very tricky so necessitates a few trips that weren’t originally planned to fit round the stabling restrictions and keep the fleet cycling while managing the cascade out of 350/2.
Long term when all the stabling is available and the fleet fully rolled out, the use will settle down to what it is meant to be. I understand there are some still specific trains throughout the week that will be 730 either for stock cycling reasons or for capacity reasons, as a 10 car 730 is equivalent to a 16 car 350.